1. Field of the Invention
The present invention relates to a display device. In particular, the present invention relates to a display device having a test circuit for simplifying testing of the display device and improving reliability, and improving a shipping yield. Further, the present invention relates to a display device having a test circuit, and relates to a correction circuit for correcting a signal which is inputted to the display device having a defect detected by the test circuit.
2. Description of the Related Art
In recent years, demand for thin displays as application mainly to TVs, PC monitors, mobile terminals, and the like has increased rapidly and further development thereof has been promoted. The thin displays include a display device using a liquid crystal element (Liquid Crystal Display: LCD) and a display device having a light-emitting element.
As an example of a display device using a light-emitting element or a liquid crystal element, an active matrix display device shown in FIG. 20 can be given.
The display device shown in FIG. 20 includes a gate signal line driver circuit 2001, a source signal line driver circuit 2002, a pixel portion 2003, and a connection terminal portion 2005 where a plurality of connection terminals 2004 are formed, over a substrate 2000. A gate line 2006 is connected to the gate signal line driver circuit 2001, while a source line 2007 is connected to the source signal line driver circuit 2002. A pixel 2008 in the pixel portion 2003 is connected to the gate line 2006, the source line 2007, and a power source line 2009. In the pixel 2008, a transistor for writing a signal from the source line 2007 to a light-emitting element or a liquid crystal element provided in the pixel, by a signal of the gate line 2006 is provided. Terminals of the transistor are connected to the gate line 2006 and the source line 2007 respectively in each pixel. Further, an FPC (Flexible Printed Circuit, which is not shown) for inputting an external signal is connected to the connection terminal portion 2005. The substrate 2000 is completed as a display module by being attached a counter substrate 2010 for sealing the light-emitting elements or the liquid crystal elements provided in the pixels 2008.
In FIG. 20, in the case of performing display in the pixel portion, providing that a Low potential of the gate line 2006 is ground, it is preferable that a Low potential of the source line 2007 be higher than the Low potential of the gate line 2006 and a difference between the Low potential of the source line 2007 and the Low potential of the gate line 2006 be equal to or higher than the threshold voltage (Vth) of the transistor for writing connected to the gate line 2006. In the case where the Low potential of the source line 2007 is lower than the Low potential of the gate line 2006 and the difference between the Low potential of the source line 2007 and the Low potential of the gate line 2006 is less than the threshold voltage (Vth) of the transistor for writing, current easily leaks from the transistor for writing so that the display device cannot perform normal display.
Note that a High potential and a Low potential of the source line and the gate line mean a relatively high potential and a relatively low potential respectively; the High potential and the Low potential may be determined so as to have a predetermined potential difference therebetween such that the High potential is a value for turning the transistor on whereas the Low potential is a value for turning the transistor off.
In the display device using the liquid crystal element or the light-emitting element shown in FIG. 20, the pixel is driven by a potential relationship between signals from the gate signal line driver circuit and the source signal line driver circuit. Therefore, it is preferable that a defect of the display device, such as that the above-described potentials cannot be held, can be detected by testing the potentials of the signals from the gate signal line driver circuit and the source signal line driver circuit.
Therefore, in the display device using the light-emitting element or the liquid crystal element, in order to perform testing for a defect of the display device, a sample in modules which had been once completed as shown in FIG. 21A has been tested using a probe 2101 of a measuring instrument after a counter substrate has been removed as shown in FIG. 21B, or testing by using a probe of an measuring instrument has been performed before a counter substrate has been attached (e.g., Reference 1: Japanese Published Patent Application No. 2002-221547)